Lasers are used to drill vias in and remove material from electronic materials products. The epoxy or resin often used in dielectric layers of electrical circuit boards is among the types of material typically removed by such a laser. For a machining laser beam to reliably and consistently remove a layer of material, it is desirable that one or both of the depth of focus and image plane of the beam fall within the depth of the layer of material undergoing removal. Variations in either the thickness or the topography of the layer undergoing removal, or in the topography of other target layers, may alter the relative location of the layer with respect to one or both of the depth of focus and image plane of the beam and thereby result in inconsistently drilled and poor quality vias.
The use of a machining laser beam and beam positioning system to drill vias in a sequentially laminated target is well known in the art. Such sequentially laminated targets typically include conductive layers and dielectric layers and are used as circuit boards in electronic circuit applications.
There are four main quality metrics that characterize a via. They include the taper of the via, the roundness of the via, the smoothness of the wall, and the cleanliness of the bottom surface. When the depth of focus of the machining laser beam is outside of the layer from which material is to be removed, vias will be drilled with nonuniform diameters. The via diameter may change 10%-20% if the layer thickness varies more than the depth of focus of the machining laser beam. When the thickness of the layer requiring material removal is small, excess power imparted by the machining laser beam can result in an over-drilled via, which exhibits one or both of poor wall quality and out-of-tolerance via size. If the layer of material is thick, insufficient power can result in incomplete via formation. Via quality is thus dependent on accurate perception of the surface height and thickness of the layer from which material is to be removed.
The state of the art for measuring the topography of a sequentially laminated target entails either touching the surface of the target with a probe and measuring its displacement or focusing a camera on a portion of the surface. Lowering and raising a probe or focusing a camera consumes an industrially significant amount of time, which elapses before the actual material removal process. Because of the time associated with current methods of measurement, the height of each target is measured only at a single location. While it permits adjustment of the depth of focus of the machining beam based on variations from target to target, a single measurement does not account for variations in topography of a single target.
Although the thickness of a single layer of material may vary by only 6 microns, the height of a target surface can vary by more than 60 microns. Since some layers requiring material removal can be as thin as 25 microns, the variation in surface height of the target is more than sufficient to cause the depth of focus to fall outside of the target layer and thereby reduce the quality of any via drilled in that layer. As technology continues to demand miniaturization, vias will likely continue to shrink in diameter, depth, or both, and, therefore, be formed by lasers of shorter (e.g., UV) wavelengths. At smaller dimensions, increased quality and repeatability are even more vital to the proper functioning of vias.
Variations in the thickness of the layer from which material is to be removed may also reduce the quality of vias formed in the layer. When the thickness of the layer is unknown, excessive or insufficient amounts of energy may be applied by the machining laser beam during material removal, leading to either damage to the underlying conductive layer or an incompletely drilled via.